We had this happen on numerous computers in the Garden last week, but only
with Firefox (not Chrome or IE). Very difficult to remove. I believe much
of the problem resides with the Addon 'Shockwave Flash' (Flash is
notoriously bad in this way). I first had to use CCleaner (freeware
http://download.cnet.com/CCleaner/…) and then
disable to Addon. You may also have to clean up with programs like Spybot
and Mawarebyte (both freeware).

Paul